If you've recently started exercising, then you may just be holding on to water (google DOMS- delayed onset muscle soreness). Also, are you getting enough calories? The appetite suppressing nature of low carb sometimes means you eat too little for your activity amount.

Another thing to consider - and this is just another theory:

I've recently read "The Primal Blueprint" By Mark Sisson, and I think maybe that my stall last year was a result of too much/too hard exercise. While you SHOULD be moving every day, Sisson posits that you should do quite a bit of exercise at 55%-75% of maximum heart rate, lift a few days a week, and all out sprint occasionally. He claims that "Chronic Cardio" actually ilicits a constant stress response from the body that can, among other things, stall weight loss. marksdailyapple.com has a lot more information.

Personally I've gone from 30-60 minutes hard swimming 5X per week and intense yoga (it's really intense at my weight) 2X per week to light "water play" 3 to 4 times a week, walking as many days as possible and occasional sprinting and lifting to see if it makes a difference.